Radio-relay stations (RRS) of the millimeter wave band are widely used in communication networks to provide a wireless connection of the point-to-point topology between two remote network nodes. Typical application fields for RRS of the millimeter wave band are connections of relatively short ranges but requiring high data rates approaching several gigabits per second and tens of gigabits per second. High data rates are provided by using high bandwidths of a transmitted signal ranged from hundreds of megahertz to several gigahertz.
Efficient signal transmission in RRS requires a sufficient level of the received signal power. This requirement can be transformed to a necessity for a sufficient effective antenna aperture exceeding at least 100 cm2 irrespectively of a wavelength. For millimeter wave systems such effective aperture includes a lot of wavelength squares which translates to a high antenna gain and a sufficiently small effective width of the main radiation pattern lobe. The said feature is reflected also in a known Friis equation:G=G1G2(λ/4πd)2,
where G is a total power gain of a channel, G1 and G2 are gains of transmitting and receiving antennas, λ is a signal wavelength, d is a distance between a transmitter and a receiver. As follows from the equation, if the wavelength is decreased the antenna gains are to be increased in order to keep the total channel gain unchanged.
A typical width of the main radiation pattern lobe for RRS of the millimeter wave band used in practice does not exceed 1-3° measured at the level of −3 dB relative to the maximum. For less apertures and greater widths of the main radiation pattern lobe, the total channel power gain turns out to be so low that RRS lose their practical applicability. A width of the main radiation pattern lobe sets an upper limit to an acceptable value of antenna orientation errors in each of the two planes: azimuth and elevation. In particular, assuming the maximal acceptable loss in the total channel power gain of 3 dB and considering presence of angular orientation errors at the two link ends and in the two planes, each orientation error can cause an antenna gain loss in the line-of-sight direction of no more than 0.75 dB. For a typical width of the main lobe at the level of −3 dB of 2° this requirement limits the range of acceptable angular errors by the value of ±0.5°.
The prior art includes methods for increasing the effective angular coverage of an antenna while keeping the same gain due to a possibility of varying the radiation pattern. This possibility reduces requirements to the angular antenna orientation accuracy. An example of such method is disclosed in U.S. Pat. No. 9,391,688, published Aug. 14, 2014, “System and method of relay communication with electronic beam adjustment”, a prototype. The patent proposes a method for changing a radiation pattern of a narrow beam antenna during system operation to provide permanent alignment of the line-of-sight direction and the direction of a modified radiation pattern maximum. However, in the proposed method the scanning range, i.e. a range of variability of a direction of the radiation pattern maximum, turns out to be limited. It is known from the antenna theory that a ratio of the angular scanning range to the width of the main radiation pattern lobe at the level of −3 dB cannot exceed the number of independently controlled radiating elements in a corresponding projection (vertical for the elevation angle and horizontal for the azimuth angle). On the other hand, in RRS of the millimeter wave band the total number of independently controlled elements is equal to the number of independent radio frequency channels supplying the antenna and, in the majority of cases, to the number of radio frequency modules within a system. The number of such channels and modules is practically limited by an order of 10 because of implementation issues. Considering, for example, 9 channels, the number of elements in each projection is equal to 3 and therefore the maximal scanning range is only three times more than the width of the main radiation pattern lobe at the level of −3 dB. For the example discussed above, usage of the mentioned system changes the required initial antenna orientation accuracy from ±0.5° only to ±2.5° ideally.
Simplest methods for mechanical setting the antenna orientation not requiring special devices can provide errors of setting the angle of about ±3° and more. For RRS of the millimeter wave band this accuracy often becomes insufficient for antennas with both a fixed radiation pattern and a steerable direction of the maximum. Therefore, the problem of accurate mechanical adjustment of the antenna orientation using special technical means (during mounting or during RRS operation if it is required) for RRS of the millimeter wave band is still actual.
The prior art includes different methods for such adjustment which can be conventionally divided into methods assuming usage of additional devices and methods utilizing the RRS transceivers themselves for selection of an orientation angle. Besides, methods for antenna orientation adjustment utilizing the transceivers have a significant advantage. U.S. Pat. No. 7,501,982, published, “Antenna alignment method”, a prototype, discloses a method for antenna orientation selection utilizing RRS transceivers. The proposed invention assumes a support of a special operation mode for antenna orientation adjustment by an RRS. In the adjustment mode receiver sensitivity is increased via one of numerous methods, e.g. via transmission of special long sequences of a known signal and applying the respective digital signal processing methods at the receiver. It is assumed that the adjustment mode provides a capability to perform channel power gain measurements (link power budget measurements) even if the line-of-sight direction is aligned with a direction of side lobes of the radiation pattern. Performing mechanical antenna orientation adjustment for maximizing the measured power gain is assumed.
The described method enables informing a rotary device or staff performing antenna orientation adjustment about a value of the required angular rotation only within the angular range of the main radiation pattern lobe. For greater deviations of the antenna orientation angle leading to alignment of the line-of-sight direction with directions of side lobes, a relation between the channel gain and the orientation angle becomes ambiguous and therefore a value of the current rotation angle cannot be determined. The described method also does not provide a possibility to determine a direction of the required angular correction because the majority of antennas have symmetrical radiation patterns. Besides, for antennas having low levels of side lobes or for long haul lengths gain measurements out of the main radiation pattern lobe can be impossible due to inability of signal detection and performing synchronization over the thermal noise. Therefore, application of the invention assumes performing a blind search of the RRS antenna optimal orientation.
Accelerating a process of selection of the angular orientation of an RRS antenna requires a method of mechanical antenna orientation adjustment not requiring application of additional equipment and providing a capability for indication of a current value and a direction (a sign) of the angular orientation error at each time instant avoiding a blind search of the optimal antenna orientation.